Re: Popping key causes dict derived from object to revert to object

Mark Bourne <nntp.mbourne@spamgourmet.com> writes:

Loris Bennett wrote:

Hi,
I am using SQLAlchemy to extract some rows from a table of 'events'.
 From the call to the DB I get a list of objects of the type
   sqlalchemy.orm.state.InstanceState
I would like to print these rows to the terminal using the
'tabulate'
package, the documentation for which says
   The module provides just one function, tabulate, which takes a
list of
   lists or another tabular data type as the first argument, and outputs
   a nicely formatted plain-text table
So as I understand it, I need to convert the InstanceState-objects
to,
say, dicts, in order to print them.  However I also want to remove one
of the keys from the output and assumed I could just pop it off each
event dict, thus:
               event_dicts = [vars(e) for e in events]
     print(type(event_dicts[0]))
     event_dicts = [e.pop('_sa_instance_state', None) for e in event_dicts]
     print(type(event_dicts[0]))

>
vars() returns the __dict__ attribute of the object.  It may not be a
good idea to modify that dictionary directly (it will also affect the
object), although it might be OK if you're not going to do anything
else with the original objects.  To be safer, you could copy the event
objects:
    event_dicts = [dict(vars(e)) for e in events]
or:
    event_dicts = [vars(e).copy()]

Thanks for making this clear to me.  However, in the end I actually
decided to use the list comprehension without either 'dict()' or
'vars().  Instead I just select the keys I want and so don't need to pop
the unwanted key later and can simultaneously tweak the names of the
key for better printing to the terminal.

However, this prints
   <class 'dict'>
   <class 'sqlalchemy.orm.state.InstanceState'>
If I comment out the third line, which pops the unwanted key, I get
   <class 'dict'>
   <class 'dict'>
Why does popping one of the keys cause the elements of the list to
revert back to their original class?

>
As Dieter pointed out, the main problem here is that pop() returns the
value removed, not the dictionary with the rest of the values.  You
probably want something more like:
    for e in event_dicts:
        del e['_sa_instance_state']
(There's not really any point popping the value if you're not going to
do anything with it - just delete the key from the dictionary)

Yes, I was mistakenly thinking that the popping the element would leave
me with the dict minus the popped key-value pair.  Seem like there is no
such function.

Cheers,

Loris
 
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